Operation of switched mode or resonant energy converters in self-oscillation mode is of general, and increasing, interest due to the relatively low power loss of this mode of operation. In this mode, the timing of the switch or switches is controlled not by an external timer or clock, but by a resonant characteristic of the circuit itself. For efficient operation of the device, it must be able to sense the maxima and minima, or peaks and valleys, in an appropriate characteristic such as an output current or an inductor current.
For self-oscillation mode, an effective valley-detection circuit is important to ensure a low power loss. Known valley-detection circuits usually have direct valley detection; that is to say, the circuit detects a minimum—which corresponds to the bottom of a valley—in the signal. Such circuits include TEA1750 and TEA1507 from NXP Semiconductor's Greenchip families. However, direct detection is non-ideal as will be discussed below. Furthermore, there is a trend towards higher conversion frequencies; a conversion frequency of 1 MHz can result in a ringing frequency of up to 4 MHz. At these frequencies the disadvantages of direct detection become more significant. The increasing commercial and technical drive towards higher frequencies is due in part to the fact that this enables smaller and cheaper passive components such as inductors and capacitors, as well as other advantages for particular applications such as better colour stability and increased dimming range for lighting applications.
In direct detection circuits the level of ringing is measured without any error feedback. An example of a direct detection circuit for an energy converter is given in International Patent Application Publication WO01/78467A1.
An exemplary self-oscillation boost mode converter incorporating direct detection is shown in FIG. 1, and the relevant voltages and currents are shown in FIG. 2. FIG. 1 shows switch 1, the source of which is connected to sense resistor 2 that is used to sense inductor current, and define the maximum coil current. Inductor 3 is in series with capacitor 4 and forms a resonant tank with the capacitor 4. The converter has a primary stroke during which the switch 1 is turned on and current through the inductor increases. Upon detection of a set peak current value Ipeak through the inductor 3, the switch 1 is turned off. Thereafter the diode 5 is conducting, and the inductor discharges through diode 5 to the output. On detection of minimum drain voltage during ringing period Tr, the switch 1 is turned on again to restart the cycle
Detection of the peak inductor current Ipeak can be implemented in various ways. The example shown in FIG. 1 includes the sense resistor (Rs) 2 in series with the source of the switch 1. Consequently Ipeak is determined from Vpeak/Rs. Alternatively, peak current detection can be implemented with a sense resistor in series with inductor 3.
A common implementation of zero current (Izero) detection is to detect the voltage valley on the switch node 6, that is, the drain of the switch. This utilises the resonant tank of the inductor 3 along with capacitance 4 present on the switch node. An advantage of this detection method is that switching losses and electromagnetic interference are minimised because the voltage on the inductor will be softly reversed and the switch will turn on when the switch node voltage achieves a minimum or valley.
The turn on moment of the switch 1 is based on direct valley detection: the ringing in the resonant tank is converted into a resonant current (Is) through a capacitor (Cs) 7. The current Is is mirrored in current mirror 9, and the level of the mirrored resonant current is evaluated during every conversion cycle by a current comparator 8 which comprises current sources Ic (8′) and a buffer stage 8″. Combined with some logic control 10 the comparator output is used to turn on the switch through driver 11.
For such a direct detection method, the accuracy is limited by                1. delay of the driver: the driver is used to drive the converter switch, and its delay will be directly added to any phase error;        2. delay of the comparator 8: the delay of the comparator will also be added to the phase error.        
Thus, valley detection based on direct detection is inherently inaccurate. For low frequency operation of a converter, this in-accuracy may be acceptable. However, for higher ringing frequencies of up to and around 4 MHz, the phase error (ΔT/Tring in FIG. 3) becomes significant and can give rise to a considerable loss of power. Furthermore, due to process spread and temperature variation, both the amplitude and the frequency of the ringing will spread. This will give rise to a spread in the phase error.
As a result of these errors the practical timing diagram for a circuit such as FIG. 1 is as shown in FIG. 3, which may be compared with the ideal timing diagram shown in FIG. 2.
FIG. 2 shows the current IL through the inductor 3, the non-zero part of the current ID through the diode 5, and the voltage Vdrain at the switch node 6 for ideal operation of the boost converter of FIG. 1. During the primary stroke T1 the inductor current rises until it reaches a peak value Ipeak. At this point the switch 1 is turned off and the drain voltage (Vdrain) start to rise. After a time Tg the drain voltage achieves the output voltage (Vout). Neglecting the small voltage drop across diode 5, at this point the diode 5 starts to conduct. As the inductor 3 discharges through the diode 5 the inductor current and diode current will fall. When the inductor current crosses zero the drain voltage (Vdrain) will start to fall. This is the end of the discharge period T2, and the start of a ringing period Tr. In the ideal situation depicted in FIG. 2 the end of the period Tr occurs when the drain voltage (Vdrain) reaches a minimum value, which is determined solely by the resonant frequency of the resonant tank formed from the inductor 3 and capacitance at switch node 6. For this to be the case, the ringing period Tr would last for half the resonant cycle time of the resonant tank; that is to say, Tr=Tring/2. At this moment, in the ideal case shown, the switch 1 is switched on and the drain voltage immediately falls to zero.
However, for a practical device, as shown in FIG. 3, the period Tr is likely to be longer than half the resonant cycle time of the resonant tank (which is shown in the diagram as Tring/2) by reason of the errors discussed above. In FIG. 3 the difference between the period Tr and half the resonant cycle time of the resonant tank (Tring/2) is shown as the period ΔT. During this period ΔT the drain voltage is rising due to the oscillation of the resonant tank, and the inductor current is rising above zero. Thus when the switch 1 is turned on, there is a an extra loss associated with the non-minimum drain voltage of the switch 1.
An alternative method of realising a valley detection circuit is disclosed in WO01/78467, and illustrated in FIG. 4. In this disclosure the ringing is converted into a resonant current (Is) through the capacitor (Cs), and the peak of the resonant current (Is) is detected in peak current detector 41. Then, the threshold value of the current comparator is derived by multiplying the peak value of Is with a constant K, where K has a value between zero and 1. Using adaptive feed forward control comprising comparator 43, the delay due to the comparator and driver 44 can be compensated by carefully adjusting the value of K in the design. However, the accuracy of this valley-detection circuit will be sensitive to the spread of the ringing frequency and the spread of the comparator and driver delay.
The ideal timing circuit for a feed-forward control circuit such as that of FIG. 4 is shown in FIG. 5, which shows the inductor current, IL, the scaled peak value (Isp) of Is, that is Kx Isp, and the drain voltage Vdrain.
There is thus a continued need for an energy converter which has an improved valley detection control mechanism.